Owing to its almost unlimited resources and harmlessness, carbon is an excellent material in view of resources and environmental problems. Carbon materials have diverse forms of interatomic bonding, and a variety of crystal structures are known, such as diamond, diamond-like carbon, graphite, fullerene and carbon nanotubes. Above all, diamond-like carbon having an amorphous structure (amorphous carbon) is expected to be applied in each industrial field, because of its high mechanical strength and good chemical stability. However, general amorphous carbon films have an electric resistance in a range of semiconductors to insulators. In order to further widen the use of amorphous carbon, it has been requested to impart electric conductivity to amorphous carbon.
Examples of a process for imparting electric conductivity to amorphous carbon include a process for adding a metal to amorphous carbon. For example, PTL 1 discloses a carbon film containing metal elements and including clusters having a graphite structure in an amorphous structure. The clusters are formed in areas surrounding the added metal. However, the added metal may become a cause of corrosion or, when used in contact with another metal, may become a cause of adhesion, so inherent chemical stability of amorphous carbon can be damaged.
On the other hand, in PTL 2, electric conductivity is imparted to amorphous carbon without adding metal. PTL 2 discloses a carbon film having a structure in which sp2-bonded crystals having sp2-bonded carbon within part of the crystal extend continuously from a lowermost layer (a substrate side) to an uppermost layer (a surface side) of the film in a film thickness direction and other portions than the sp2-bonded crystals are amorphous. According to the description of PTL 2, a reduction in the content of sp2-bonded crystals is important in view of corrosion resistance and wear resistance of the carbon film. Therefore, it is desirable that the sp2-bonded crystals extend continuously from the substrate side to the surface side of the carbon film, because it is effective in increasing electric conductivity in a film thickness direction of the carbon film and as a result can reduce the content of the sp2-bonded crystals. Moreover, PTL 2 also states that an increase in the content of the sp2-bonded crystals causes a decrease in hardness and wear resistance of the carbon film. That is to say, PTL 2 suggests that the amorphous portions of the carbon film contain a large amount of spa-bonded carbon, which improves wear resistance and hardness.
Moreover, the present inventors found that electric conductivity can be imparted to an amorphous carbon film by increasing the amount of carbon having an sp2 hybrid orbital and invented amorphous carbon recited in PTL 3. Depending on a difference in atomic orbital in chemical bonding, there are three types of carbon atoms: carbon having an sp hybrid orbital (Csp), the abovementioned carbon having an sp2 hybrid orbital (Csp2), and carbon having an spa hybrid orbital (Csp3). For example, diamond, which is composed of Csp3 only, forms σ bonds only and exhibits high electric insulation due to localization of a electrons. On the other hand, graphite is composed of Csp2 only and forms a bonds and π bonds and exhibits high electric conductivity due to delocalization of π electrons. In the amorphous carbon film recited in PTL 3, delocalization of π electrons is promoted due to a large content of Csp2 in the entire carbon and molecular termination by C—H bonds (a bonds) is suppressed due to a decrease in hydrogen content. As a result, the carbon film of PTL 3 exhibits a high electric conductivity.